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erecting prism for Newtonian Reflector question

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#1 thinairflyer

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Posted 15 September 2018 - 05:42 PM

Does anyone have any experience with the straight erecting prisms such as the one on Ebay and Amazon labeled Datyson?  Supposed to be an erecting prism at 1.5X power.

 

Wondering how good the image is with this item.



#2 dmgriff

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Posted 16 September 2018 - 11:57 AM

There are some threads about erecting eyepieces/barlows for newtionans in the forum. Those mentioned are usually inexpensive.  Many members had bad comments on the quality and concept.

 

I would not call the Datyson version inexpensive at about 50usd. So, for that money one would surely expect it to perform well.

https://www.amazon.c...l/dp/B0768YB6L8

 

However, it is a fraction of the cost of this vixen version at 150usd.

https://www.vixenopt...pter-p/3847.htm

 

Good viewing,

 

Dave


Edited by dmgriff, 16 September 2018 - 12:04 PM.

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#3 Jon Isaacs

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Posted 16 September 2018 - 12:05 PM

Does anyone have any experience with the straight erecting prisms such as the one on Ebay and Amazon labeled Datyson?  Supposed to be an erecting prism at 1.5X power.

 

Wondering how good the image is with this item.

Hello and :welcome: To Cloudy Nights.
 

It uses a roof prism so I would not expect a sharp image at higher powers.

Jon



#4 dan_h

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Posted 17 September 2018 - 08:18 PM

A Newtonian telescope presents an image that is reversed left to right but is correct top to bottom. This is the end result of the reflection the secondary mirror provides. The actual orientation of the image of course depends on how the focuser is aligned to the target.  When the focuser is on the side of the tube, top to bottom becomes left to right and this changes when the focuser is moved to the top of the tube. Either way, you do not end up with a correct image.

 

While these image erectors do work on refractors to provide fully correct image orientation in the straight through mode, a full erecting prism used on a Newtonian telescope will create a final image that is not correct.

 

dan



#5 RobertMaples

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Posted 17 September 2018 - 10:14 PM

A Newtonian telescope presents an image that is reversed left to right but is correct top to bottom..

Nope, a Newtonian flips the image in horizontally and vertically.



#6 25585

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Posted 18 September 2018 - 04:30 AM

https://www.bhphotov...rect_Image.html



#7 dan_h

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Posted 18 September 2018 - 04:20 PM

Nope, a Newtonian flips the image in horizontally and vertically.

Horizontally and vertically?  Yes, the image from the primary mirror is fully inverted but then the secondary flips it in one axis. Whether this be the horizontal or the vertical (wrt the target) depends on whether the focuser on the Newt is placed on the side of the tube or the top of the tube.  Either way, the secondary corrects the image around one axis only. If the image is then put through a fully erecting corrector, then it will be reversed around that one axis. 

 

A Newtonian is a little weird for terrestrial viewing no matter what because of the different possible focuser positions and because of the different stances the observer can choose.  It just never seems right. 

 

dan



#8 RobertMaples

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Posted 18 September 2018 - 08:03 PM

Horizontally and vertically?  Yes, the image from the primary mirror is fully inverted but then the secondary flips it in one axis. Whether this be the horizontal or the vertical (wrt the target) depends on whether the focuser on the Newt is placed on the side of the tube or the top of the tube.  Either way, the secondary corrects the image around one axis only. If the image is then put through a fully erecting corrector, then it will be reversed around that one axis. 

 

A Newtonian is a little weird for terrestrial viewing no matter what because of the different possible focuser positions and because of the different stances the observer can choose.  It just never seems right. 

 

dan

Yes, horizontally and vertically.  Actually, you could say that it flips it in an infinite number of axes, because the light path from any two opposite edges of the view will cross at the focal point.  The primary mirror in a Newtonian has two effects on the image, a mirroring effect and a focusing effect.  The secondary cancels out the mirroring effect, but you still have the focusing effect, which flips the image in both axes.  With the eyepiece horizontal (which should be pretty easy to maintain for terrestrial viewing), you get the same view in a Newtonian that you get with a refractor with no diagonal or erecting prism.


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#9 JeffreyAK

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Posted 19 September 2018 - 09:42 AM

Mirrors invert in depth, not side to side or vertically, is the way to think about it.  When you look in a flat mirror, many people say it's inverted left/right but not up/down, but that's not correct - when you point your finger to the left, your image does too (your left, not it's left), and when you point your finger up, your image does too.  It only gets confusing when you imagine yourself taking the place of your image and keeping your head pointed in the same direction - in which case up is still up, but left and right swap.

 

A Newtonian is essentially a refractor with a mirror replacing the objective.  The refractor produces an inverted image (left/right and up/down), but the primary mirror adds a depth inversion, which is cancelled by the secondary.  So it's still inverted.  If you were to look down the tube at the primary with an eyepiece (imagine you have a transparent head), it's still inverted - if you point left, that's right in the image - and it only gets confusing if you then imagine yourself rotating about your head and thinking about what the scene would look like.  In that case left/right swap, and the scene appears flipped up/down but not left/right.



#10 dan_h

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Posted 22 September 2018 - 10:11 AM

Mirrors invert in depth, not side to side or vertically, is the way to think about it.  When you look in a flat mirror, many people say it's inverted left/right but not up/down, but that's not correct - when you point your finger to the left, your image does too (your left, not it's left), and when you point your finger up, your image does too.  It only gets confusing when you imagine yourself taking the place of your image and keeping your head pointed in the same direction - in which case up is still up, but left and right swap.

 

A Newtonian is essentially a refractor with a mirror replacing the objective.  The refractor produces an inverted image (left/right and up/down), but the primary mirror adds a depth inversion, which is cancelled by the secondary.  So it's still inverted.  If you were to look down the tube at the primary with an eyepiece (imagine you have a transparent head), it's still inverted - if you point left, that's right in the image - and it only gets confusing if you then imagine yourself rotating about your head and thinking about what the scene would look like.  In that case left/right swap, and the scene appears flipped up/down but not left/right.

 The "mirrror effect" and "depth inversion" are characteristics of the virtual image seen in a plane mirror such as one would find in a dressing room or bathroom.  These characteristics have nothing to do with the real image formed by a parabolic mirror such as a Newtonian primary.

 

A parabolic mirror forms an image identical to what a refractor objective of the same focal length would create. Both images are fully inverted, top to bottom and left to right. The diagonal in the Newtonian performs the exact same function as the diagonal used with a refractor, and that is to deflect the image to the side for a more convenient viewing position.  In both types of scopes, the diagonal corrects the image in one plane only, be that up/down or left/right, depending on the orientation of the diagonal, vertical or horizontal.  

 

There are reflecting telescopes that mount cameras and instrumentation, (or an observer), directly at the primary image and do not use a secondary mirror. 

 

dan 



#11 JeffreyAK

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Posted 22 September 2018 - 11:43 AM

Maybe this will help explain what I meant.  If we ignore inversions that can be corrected by rotations, and we always rotate our heads about the eyepiece axis so that up is up, then all that matters is the handedness of the view, i.e. can we read text written on the object from left to right, same as we would looking at the object?  If we can, then the view is unchanged, but if we can't then the handedness is flipped, e.g. through a depth inversion by a mirror (obviously we can do this in software too, with electronic images).

 

From this perspective, the view through a refractor is unchanged - it's exactly what we'd see with our eyes looking at the same object, except it's magnified.  Text can be read normally from left to right.  Yes, we'd have to stand on our heads to achieve this, but we'll ignore this because in principle we could.

 

Replacing the objective lens with a parabolic mirror and looking backwards through your transparent head is *not* the same, because now the handedness of the image is flipped by the parabolic mirror, just like the bathroom mirror.  If I rotate my head so that up is up, then I find I can't read text on the object.  So the parabolic primary focuses like the refractor objective, but it does something different too, it flips the handedness of the view like the bathroom mirror.

 

Adding a secondary mirror flips the handedness again, back to normal, so the view through a Newtonian is identical to the view through a refractor.  Once I deal with the rotations and make sure up is up, I can read text normally.



#12 dan_h

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Posted 22 September 2018 - 12:10 PM

Maybe this will help explain what I meant.  If we ignore inversions that can be corrected by rotations, and we always rotate our heads about the eyepiece axis so that up is up, then all that matters is the handedness of the view, i.e. can we read text written on the object from left to right, same as we would looking at the object?  If we can, then the view is unchanged, but if we can't then the handedness is flipped, e.g. through a depth inversion by a mirror (obviously we can do this in software too, with electronic images)............

 

 

.........Adding a secondary mirror flips the handedness again, back to normal, so the view through a Newtonian is identical to the view through a refractor.  Once I deal with the rotations and make sure up is up, I can read text normally.

Clearly you have some very firm beliefs about how a telescope forms an image.  Thank you for your viewpoint. 

 

dan


Edited by dan_h, 22 September 2018 - 12:16 PM.


#13 stargazer193857

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Posted 22 September 2018 - 06:39 PM

Horizontally and vertically? Yes, the image from the primary mirror is fully inverted but then the secondary flips it in one axis. Whether this be the horizontal or the vertical (wrt the target) depends on whether the focuser on the Newt is placed on the side of the tube or the top of the tube. Either way, the secondary corrects the image around one axis only. If the image is then put through a fully erecting corrector, then it will be reversed around that one axis.

A Newtonian is a little weird for terrestrial viewing no matter what because of the different possible focuser positions and because of the different stances the observer can choose. It just never seems right.

dan

Instead of theorizing and then stating your theory as fact multiple times, how about looking through a newtonian. You will see that it flips both axis. The image is rotated 180 degrees.

...
I just went outside and checked. It flips both axis the same.

I do think a focuser has room for an abbe-konig prism, but I can pan around just fine with reversed images. Having a correct image finder may change that.

...

Now, changing the angle of the focuser does rotate the image and make tracking less intutive, but both axis are affected the same.

Edited by stargazer193857, 22 September 2018 - 06:57 PM.


#14 25585

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Posted 23 September 2018 - 12:14 PM

Maybe the Orion erecting bino viewer is what might interest the OP?  https://uk.telescope..._campaign=UK-cj



#15 Starman1

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Posted 27 September 2018 - 05:55 PM

Does anyone have any experience with the straight erecting prisms such as the one on Ebay and Amazon labeled Datyson?  Supposed to be an erecting prism at 1.5X power.

 

Wondering how good the image is with this item.

They are not usable on newtonians because newtonians don't have enough in-travel at the focuser to achieve focus with one in place.

Many years ago, you could buy a lens-based erecting device for the focuser, but these were crude and poor optically.

The truth of the matter?  Newtonians are simply not terrestrial scopes.

What you need is a small refractor for that purpose.


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#16 RobertMaples

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Posted 28 September 2018 - 09:07 AM

They are not usable on newtonians because newtonians don't have enough in-travel at the focuser to achieve focus with one in place...

The one the OP is talking about is a barlow and erecting prism in one and doesn't look like it would add any more length than a regular barlow, so if you can use a barlow, I would think you could use one of these.  Also, considering the Amazon and ebay pages for the different versions (it looks like the exact same thing is sold under several different names) all say "for Newtonian reflector telescopes," I would find it odd if they would not work with them.

 

Anyway, I've ordered one and I'll let everyone know how it works when it gets here, but it may be a while since it's shipping from China.


Edited by RobertMaples, 28 September 2018 - 09:08 AM.


#17 Starman1

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Posted 28 September 2018 - 09:09 AM

Good luck.  The ones I've tried of that type require >1" in travel at the focuser.


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#18 Jon Isaacs

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Posted 28 September 2018 - 09:36 AM

Good luck.  The ones I've tried of that type require >1" in travel at the focuser.

 

I had an older 1.25 inch Orion Image corrector for a number of years.  It was made in Japan so one would assume that it was of reasonably high quality.  I was never impressed with the views and if I remember correctly, it had clear aperture issues as one might expect.  I gave it to a friend to sell.. 

 

3908934-Orion Image Corrector.jpg

 

Jon



#19 thinairflyer

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Posted 28 September 2018 - 11:09 AM

I just, on an Orion XT8 Reflector, (clean mirror and recent collimation) started with a 2X Barlow, followed by a 90 degree erecting prism turned to the rear and then a 40mm or 26mm eyepiece.  Focused on a 14,000' peak 18 miles away.  Standing to the side of the scope and looking forward into the eyepiece gives a fully erect image.

 

I will try this on a closer target and see how the light and higher power eyepiece selection works.

 

There is a fairly significant loss of light with this setup (all entry level items).

 

Perhaps with a higher quality erecting prism and Barlow light loss would be somewhat less.

 

Focuser was near midrange.



#20 Jon Isaacs

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Posted 28 September 2018 - 04:59 PM

Perhaps with a higher quality erecting prism and Barlow light loss would be somewhat less.

 

Erecting prisms have a complicated light path that splits the beam in two and then recombines them.  Ok at lower magnifications, not so good at higher magnifications.

 

Jon



#21 stargazer193857

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Posted 29 September 2018 - 10:58 AM

Even if you put an Abbe Konig into a newtonian focuser tube, the image rotates with the rotation of the tube. I have the focuser 30 degrees from horizontal. The moon tress then are rotated 30 degrees. The public would definitely notice this on the moon. A horizontal focuser is pure 180 degrees, acceptable on the moon, but also no easy to look into. My conclusion is to use big scopes on galaxies and upright views for the moon.

#22 RobertMaples

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Posted 21 October 2018 - 08:45 PM

So, I finally got chance to test out the erecting prism (I got it about a week ago).  It my opinion it works quite well, nothing like the crude lens based plastic ones that some cheap telescopes come with.  The only real issue found, depending on the desired magnification, is the barlow is giving about 2.4x instead of the advertised 1.5.  As someone mentioned, if the focuser is not horizontal, then the image will be rotated the same number of degrees as the focuser.  If viewing seated, it should be easy to have the focuser horizontal; however, if standing it may be hard to have the telescope high enough.  I was using it standing with the focuser tilted up and I just stood slightly to the side so I was looking in the eyepiece at a an angle and it actually worked quite well.  

 

Here's the erecting prism I bought ($30 off of ebay):

20181021_200942.jpg

 

Here's the view of a transformer on a utility pole using a 20mm eyepiee (114mm/450mm fl newtonian):

20181021_101044.jpg

 

And a view of the same transformer through the same eyepiece with the erecting prism:

20181021_102101.jpg


Edited by RobertMaples, 21 October 2018 - 08:45 PM.


#23 thinairflyer

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Posted 22 October 2018 - 12:32 AM

I recently bought the Vixen upright adapter.  Expensive, but works extremely well.  No prism, just three lens.

 

Very very minimal light loss, very sharp images, and zero magnification.

 

The only drawback if it is one, depending on its use, is that it is about 7" long.  That puts the operator quite a distance back from a refractor as the draw tube will be near its rearward travel limit using the Vixen.  This should not be a problem on a reflector.


Edited by thinairflyer, 22 October 2018 - 12:35 AM.


#24 dmgriff

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Posted 22 October 2018 - 10:45 AM

Glad you have found one that works for you.

 

Usually, anything labeled Vixen is not "inexpensive", but, they tend to do what they are supposed to do.

 

Good viewing,

 

Dave



#25 csrlice12

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Posted 22 October 2018 - 11:12 AM

The only question left is Why?  




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